As one moves from left to right across a period in the periodic table, the atomic radius of elements generally decreases. Which of the following statements provides the most accurate explanation for this observed trend?
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The answer
Correct answer: A. The increasing effective nuclear charge pulls the valence electrons more strongly towards the nucleus, reducing the atomic size.
Tested concept: Periodic trends: atomic radius
Explanation
Across a period, electrons are added to the same principal energy level. While the number of electrons increases, the number of protons in the nucleus also increases, leading to a greater effective nuclear charge. This stronger positive charge from the nucleus pulls the valence electrons more tightly inward, resulting in a decrease in atomic radius. The option stating that the number of electron shells increases is incorrect because the number of electron shells remains constant across a period; it increases down a group. The option suggesting that repulsion between increasing electrons in the same shell causes the cloud to expand is incorrect because while electron-electron repulsion exists, the increasing effective nuclear charge is the dominant factor determining the trend in atomic radius across a period. The option claiming that the shielding effect of inner electrons significantly increases is incorrect because the shielding effect from inner electrons does not significantly increase across a period for valence electrons in the same shell; it is relatively constant.
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